In broad terms, electrosurgery is the application of a high-voltage, high-frequency (HF) or radio-frequency (RF) output waveform to tissue to achieve a surgical effect. Tissue is cut, coagulated by stopping blood flow, or simultaneously cut and coagulated, depending upon the electrical characteristics of the electrosurgical output waveform. To achieve cutting, the output signal is substantially continuous. To achieve coagulation, the output signal is delivered in bursts with each burst defined by a duty cycle in which the on-time of the duty cycle is substantially less in time duration than the off-time. To achieve simultaneous cutting and coagulation, the output signal is also delivered in bursts, but the on-time and the off-time of the duty cycle are comparable in time to each other, or the on-time may exceed the off-time. The electrosurgical output signal is delivered to the tissue from an active electrode of an applicator or handpiece that is manipulated by the surgeon. The electrosurgical output signal is conducted to the active electrode over a conductor extending from the electrosurgical generator to the applicator or handpiece.
Accurate knowledge of the voltage and current characteristics of the electrosurgical output signal is important for controlling and monitoring the power applied to the tissue, or for monitoring other parameters related to or derived from the electrosurgical output voltage and current, such as impedance. The output voltage and current values may also be monitored to ensure that they stay within desired operating limits, for example. Data describing the electrosurgical output signals may also be collected when monitoring a surgical procedure.
The load into which the electrosurgical output signal is delivered varies substantially during a surgical procedure due to large and almost instantaneous changes in the point-to-point resistance or impedance of the tissue encountered. For example, a highly fluid-perfused tissue, such as the liver, may exhibit a resistance or impedance in the neighborhood of 10-20 ohms while other tissues, such as skin or bone marrow, may have an impedance in the neighborhood of 1000 to 2000 ohms. When the active electrode passes from low impedance tissue into high impedance tissue, less current is momentarily delivered to the high impedance tissue thereby immediately degrading or inhibiting the desired electrosurgical effect. On the other hand, when the active electrode passes from high impedance tissue into low impedance tissue, high current is momentarily delivered into the low impedance tissue and that high current may create excess tissue damage. The variable impedance characteristics of the tissue require the electrosurgical generator to deliver and control relatively wide variations of power on a rapidly-changing basis.
The common technique of monitoring the voltage and current of an electrosurgical output signal is to connect output voltage and current sensors to the conductor extending from the electrosurgical generator to the handpiece or applicator. These sensors develop output current and output voltage sense signals corresponding to the actual output current and voltage applied to the tissue. However, the output sensors impose additional capacitive, resistive and/or inductive loads on the electrosurgical output signal, thereby changing its output characteristics. The load from these sensors decreases the amount of power reaching the tissue and increases leakage current. Leakage current is current which flows into the surrounding environment other than into the tissue at the surgical site. Leakage current can cause a lack of accuracy in output power regulation, or be a source of unintended patient burns and hazards to operating room personnel. The output voltage sensors must have the capability of withstanding high output voltage, and therefore the sensors may be relatively costly.
Because of these and other problems with output voltage and current sensors, attempts have been made to regulate output power by using voltage and current sensors on the primary winding of a power output transformer of an electrosurgical generator. The power output transformer transforms the energy of the primary voltage and current signals supplied to the primary winding into the electrosurgical output signal which is supplied from the secondary winding of the transformer. If the power output transformer was an ideal electrical element, the voltage and current of the output signal would be directly related mathematically to the voltage and current at the primary winding by a constant multiplied by the ratio of the turns of the primary winding and of the secondary winding. However, these simple relationships are valid only for an ideal transformer, and an electrosurgical power output transformer is far from an ideal circuit element. Consequently, the voltage and current signals from the primary winding voltage and current sensors do not accurately represent the voltage and current of the output signal from the secondary winding in a typical electrosurgical power output transformer.
An electrosurgical power output transformer has a complex set of variable electrical characteristics which cause the output signal to be substantially different from the primary current and voltage signals. Parasitic capacitances between the windings and the core of the output transformer store and divert different amounts of energy at different frequencies, and therefore cause changes in the spectral energy content or bandwidth of the output signal compared to the input signals. Core losses consume energy from the primary signals, as do the resistances of the winding conductors. The core losses depend on the frequency component of the signals and the instantaneous value of the load presented by the tissue. The electrosurgical output signal, particularly in a spray coagulation mode of operation, use the energy storage characteristics of the inductance of the primary winding to delay the energy delivery to the output signal. Consequently, the relationships of the input signal to the output signal become phase shifted, frequency attenuated and frequency distorted relative to one another.
In those types of electrosurgical generators which rely on sensing the voltage and current at the primary winding of the output transformer, the sensed primary voltage and current values, which do not accurately represent the voltage and current of the output signal, are instead used as approximations. Those approximations do not provide the precision desired for accurate control and monitoring of the electrosurgical output signal applied to the patient.
Even in those cases where the output signals are sensed by output sensors, the signals supplied by the output sensors may not be processed quickly enough to be effective in precisely controlling the output power from an electrosurgical generator. A control loop time lag or phase lag, which is that time between acquiring the sensed signals and making an adjustment to the output signals, may be so long that a response cannot be achieved quickly enough to obtain or maintain the desired effect. The control loop time lag or phase lag is dependent upon many factors, but a principal factor relates to the speed at which the output voltage and current signals may be derived and processed into a usable feedback or other control signal. The same circumstance also applies with respect to monitoring the other output-related factors, such as tissue impedance, which must be calculated based on the output voltage and current signals that exist on an instantaneous basis.